1. Field of the Invention
The present invention relates to an electric motor and a recording disk drive apparatus provided with the motor.
2. Description of the Related Art
Conventionally, a hard disk drive, one of the recording disk drive apparatuses, includes a spindle motor arranged to rotate a recording disk (hereinafter simply referred to as a “motor”). The motor includes a closed-top rotor hub having thread holes arranged to use in fastening the disk. A clamper is attached to the top surface of the rotor hub. Thus, the recording disk is fixed to the rotor hub with the lower surface thereof making contact with the disk mounting portion of the rotor hub. The thread holes are defined in the form of through-holes so that the rotor hub can be easily cleansed during the course of manufacturing the motor.
In one example of such rotor hubs, a rotor magnet arranged to generate torque and a back iron arranged to reduce the loss of a magnetomotive force are attached between the rotor hub and the stator with coils. The back iron is made of a ferromagnetic material such as stainless steel or the like and is fixed to the rotor hub by bonding, press-fit, etc.
Japanese Patent Laid-open Publication No. 1989-129745 discloses a motor for a disk drive apparatus, which includes a rotor frame having a cylindrical portion and a disc-shaped portion. As recited on lines 2 through 12 of the left lower column in page 4 of Japanese Patent Laid-open Publication No. 1989-129745, the motor for a disk drive apparatus is configured such that the disc-shaped portion of the rotor frame makes contact with disk-fastening thread holes defined as through-holes. As a result, the disk-fastening thread holes are closed at one ends thereof, which prevents metal particles from infiltrating into the motor.
Japanese Patent Laid-open Publication No. 2002-199654 discloses a method for manufacturing a fluid bearing motor. Paragraph 0034 thereof reads: the motor hub 6 and the sleeve 3 are made of, e.g., aluminum, and formed into a unitary molded product 14, the front surface of which may be plated with nickel in a thickness of d1.
However, if the rotor frame or the back iron makes contact with the through-holes as disclosed in Japanese Patent Laid-open Publication No. 1989-129745, burrs are dropped from around the through-holes. This may be a cause of generating metal particles or dust. In addition, the vertical position of the back iron is deviated by the burrs thus dropped, which may lead to increased vibration of the motor. Production cost is increased if a sheet-like seal member is attached to the through-holes in the process of manufacturing the motor.